The demands on modern communication standards and on the signal quality of transmission devices are rising with the growing need for high data rates and increasing mobility. Mobile radio standards which have now become customary, such as Universal Mobile Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Wireless Local Area Network (WLAN) or Medium Rate Bluetooth, use bandwidth-efficient modulation types for transmitting high data rates both from a base station to a mobile appliance and from a mobile appliance to a base station. Examples of these modulation types are Quadrature Phase Shift Keying (QPSK), 8-Phase Shift Keying (8-PSK) or Quadrature Amplitude Modulation (QAM). With these types of modulation, what is known as a carrier signal has both its phase and its amplitude modulated in order to transmit the data.
In this context, a popular transmission device comprises a unit for baseband signal processing and a unit for radiofrequency signal processing. In this arrangement, the data for transmission are preprocessed in the baseband unit such that they can be modulated onto the carrier signal and amplified in the radiofrequency unit so as finally to be broadcast via an antenna. To modulate the data which is to be transmitted, what are known as mixers or frequency mixers are used.
FIG. 5 shows an exemplary embodiment of a conventional mixer operating on the basis of the double balanced principle. Such a mixer is also called a Gilbert mixer. The frequency mixer comprises two signal paths comprising transistors T1, T2 whose control connections form a signal input SIN1, SIN2 of the frequency mixer. The two signal paths are coupled to one another via a coupling impedance element IM1, which comprises an ohmic resistance, for example. The signal input SIN1, SIN2 can be used to supply a signal which comprises the data for transmission. In addition, a transistor T7 coupled to the transistor T1 and a transistor T8 coupled to the transistor T2 are provided which respectively form a current source, in the form of a current mirror, together with a transistor T9. A reference input IB coupled to the transistor T9 can be used to supply a reference current.
The frequency mixer also has two transistor pairs T3, T4 and T5, T6 whose control connections are coupled to an oscillator input LO1, LO2. The transistor pair T3, T4 has a first connection coupled to the transistor T1, while the transistor pair T5, T6 has a first connection coupled to the transistor T2. The transistors T3, T5 have a second signal connection connected to a first signal output connection SOT1. Similarly, second signal connections of the transistors T5, T6 are connected to a second signal output connection SOT2. The connections SOT1, SOT2 form a signal output of the frequency mixer.
In the frequency mixer shown, the signal input SIN1, SIN2 is used to supply data for mixing which control a current in the signal paths via the transistors T1, T2. The oscillator input LO1, LO2 is used to supply an oscillator signal, which is usually a radiofrequency square-wave signal. This alternately turns on the transistors T3 and T4 and T5 and T6, which routes the current through the transistors T1, T2 alternately to the signal output connections SOT1 and SOT2. At the signal output SOT1, SOT2, it is thus possible to tap off a mixed, differential output signal. On the basis of the principle illustrated, the input signal and the oscillator signal are multiplied.
Such mixers are used in a vector modulator, for example. FIG. 6 shows an exemplary embodiment of a conventional vector modulator. This comprises two frequency mixers MIX1, MIX2 which are supplied with data for transmission as vector data I, Q. In this arrangement, the data component I usually has a phase shift of 90° relative to the data component Q. The vector modulator also comprises an oscillator for producing an oscillator signal. This signal is supplied to a frequency divider which halves the frequency of the oscillator signal and at the same time derives two oscillator signals with a 90° shift relative to one another, which are output to the frequency mixers MIX1, MIX2. The output signals from the mixers MIX1, MIX2 are added and are provided at a common signal output RFOUT.
Frequency mixers may also be used in a polar modulator, as shown in FIG. 7, for example. In a polar modulator, the data for transmission are described as vectors with an amplitude component R and a phase component φ. In this exemplary embodiment, a signal processor DSP routes the phase component φ to a phase locked loop ΣΔ-PLL with a ΣΔ modulator which controls an oscillator CO. The oscillator CO may be a voltage controlled oscillator (VCO) or a digitally controlled oscillator (DCO). The oscillator produces a carrier signal which comprises the phase information for the phase component φ. The carrier signal is routed as an oscillator signal to a mixer MIX3 which is in the form of the Gilbert mixer shown in FIG. 5, for example.
The signal processor DSP also outputs the amplitude component R to an amplitude modulator AM whose output is coupled to the signal input of the mixer MIX3. The output of the mixer MIX3 is coupled to the signal output RFOUT for outputting the mixed radiofrequency signal.
With reference to FIG. 5, the mixers MIX1, MIX2, MIX3 in the exemplary embodiments shown in FIGS. 6 and 7 convert an input voltage at the signal input SIN1, SIN2 into a current via the transistors T1, T2 and the current sources formed by the transistors T7, T8, T9. The transistors T1, T2 are therefore a voltage/current converter. Since the transistors T1, T2 usually have a nonlinear characteristic, the voltage/current conversion produces nonlinear distortions in the input signal, for example in the form of harmonics.
When the converted current is actually mixed with the radiofrequency oscillator signal by means of the transistors T3, T4, T5, T6, the harmonics in the current signal may result in intermodulation products in the mixed radiofrequency signal. Particularly the third harmonic can widen the modulation spectrum and disturb the adjacent frequency channels. Hence, the signal quality of the mixed signal can be impaired, which is manifested by an increased bit error rate or an impaired error vector magnitude (EVM), for example.
In the exemplary embodiment shown in FIG. 5 for a frequency mixer, the distortions can be reduced by resistive negative feedback, the provision of a large operating current and lowering of the amplitude at the input of the frequency mixer, for example. However, the resistive negative feedback increases the thermal noise in the frequency mixer, while the gain falls. A larger operating current in the frequency mixer usually increases shot noise in a bipolar transistor or channel noise in a field effect transistor on account of the necessary increase in the size of the transistor channel. Lowering the input amplitude in turn regularly results in impairment of the efficiency of the frequency mixer.
This is problematical particularly because the thermal noise in the frequency mixer can dominate the overall noise in a transmitter.